1. Field of Invention
The present invention relates to differential pressure transmitters in general and, more specifically to a method and apparatus for providing overrange pressure protection for the transducer in a differential pressure transmitter.
2. Description of the Prior Art
Differential pressure transmitters are extensively used in process controls. Frequently they measure a small differential pressure at high static pressure levels. For example, it is common to measure a 3 PSI differential pressure at 1000 PSI static level. Most electrical transducers such as, semiconductor chips, that are capable of responding to small pressures are damaged when subjected to full static pressure on one side. In order to prevent such damage, many differential pressure transmitters incorporate a means for protecting the sensitive electrical transducer from overrange pressures. One commonly employed overrange pressure protection method uses isolation diaphragms between the electrical transducer and the process fluids. The volume between the isolation diaphragms and the transducer is filled with an incompressible liquid. It is customary to provide a mechanical back-up stop for the isolation diaphragm in order to limit the volume of the liquid transferred from the high pressure side to the low pressure side. Most pressure transducers and specifically the currently employed semiconductor pressure responsive chips are mechanically too rigid to accomodate the liquid transferred without permanent damage to the chip. Therefore, auxiliary methods have been proposed to limit the differential pressure applied to the transducer within a pressure range that will not produce a deleterious effect on the transducer.
An early auxiliary protection method is described in U.S. Pat. No. 3,058,350, issued Oct. 16, 1962, to G. S. Brown for a Differential Pressure Transmitter. In the Brown patent, the transducer is protected from overrange pressure by means of two preloaded bellows. These bellows remain undeflected when the transmitter is exposed to a differential pressure within the operating range of the transducer. However, when the differential pressure on one side of the transmitter exceeds a predetermined level determined by the preload on the associated bellow, the bellows begins to move in order to accomodate the transfer of the liquid from behind one of the isolation diaphragms or bellows to the other.
Although, the dual bellows protection system disclosed in the Brown patent produces adequate overrange pressure protection, it achieves this protection at the expense of transmitter accuracy because of the additional volume required by two bellows. Typically, silicone oil is employed as the liquid filling the volume between the two isolation diaphragms and the two bellows. As the oil undergoes temperature excursion during use of the transmitter, the oil volume changes and unevenly deflects the isolation diaphragms. The non-symmetrical deflection of the two isolation diaphragms produces an undesirable zero-offset error for the transmitter.
In order to reduce this type of inaccuracy, the fluid volume between the two isolation diaphragms should be held to a minimum. Since this volume includes the volume of the overrange pressure protection device, it follows that the volume of the protection device also should be held to a minimum. The use of the dual bellows system shown in the Brown patent does not achieve the desired minimum volume between the isolation diaphragms.
Another example of a prior art overrange pressure protection device is depicted in U.S. Pat. No. 3,756,085, issued Sept. 4, 1973, to Richard C. Hunter for Differential Pressure Transmitter Overrange Protection. In the Hunter device a magnetic breakaway coupling is employed to maintain a bellows substantially rigid during the normal operating pressure range of the differential pressure transmitter. When the force on the breakaway coupling exceeds the limit, the bellows deflects to allow the transfer of liquid from behind one isolation diaphragm to the other thereby limiting the differential pressure across the transducer to a safe level.
The use of a magnetic breakaway coupling not only requires an undersirable extra volume in order to achieve overrange protection, but also suffers from the lack of any return force to provide an automatic resetting of the protective device. In Hunter, a pressure is needed in one direction to return the magnetic breakaway coupling to its "set" position. If bidirectional protection is desired, the Hunter system suggests the use of two-magnetic breakaway couplings. This configuration further complicates the structure of the protective device, requires additional volume and pressures in two directions to provide automatic resetting of the device. From a practical standpoint, these disadvantages produce a significant deterrent to commercial use of the Hunter device. Although the previously mentioned Brown patent illustrates an overrange pressure protective device with an automatic reset feature through the use of two preloaded bellows, this feature is achieved at the sacrifice of minimum volume between the two isolation diaphragms.
U.S. Pat. No. 4,028,945, issued June 14, 1977 to G. Bergamine, illustrates an overrange pressure protection system for differential pressure transmitters that utilizes a movable bidirectionally compressible, preloaded spring positioned within an unloaded bellows. In operation, the movable spring seats against different surfaces depending upon the direction of the overrange pressure. Any inaccuracy in the location of these surfaces and the lack of repeatability of spring seating with respect to the surfaces can cause unacceptable zero-offset errors. Furthermore, the movement of the spring from one seating surface to another seating surface produces an undesirable "deadband" in the system.